Conventionally, a wireless communication device is known that is connected to a data processing device such as a PC and mediates transmission of a TCP data packet between the data processing device and a communication counterpart. A cellular telephone and a card-type terminal can be exemplified as the wireless communication device. Such a wireless communication device often can communicate by a plurality of communication systems, for example CDMA2000—1x and LTE (Long Term Evolution), and the like.
In communication by TCP, window control is employed for efficient data transfer. In the window control, a data receiver gives notification of receive buffer size (RWIN, Receive Window) to a data sender. The data sender thus can send data of a size corresponding to the RWIN without waiting for acknowledge (ACK) from the receiver. Here, if the RWIN is too small with respect to the line speed, data transfer efficiency and throughput are reduced. On the other hand, if the RWIN is too large with respect to the line speed, retransmission processing due to data loss requires time, leading to reduced throughput.
If communication quality is reduced during data reception by the data processing device, the wireless communication device can perform hand-off (hand-down) from a high-speed communication system (for example, LTE) to a low-speed communication system (for example, CDMA2000—1x). In such a case, the line speed is drastically reduced and control for appropriately changing the data transmission speed is required. For example, Patent Document 1 proposes a control method that reconfigures a maximum segment size (MSS) based on information (maximum throughput) of a communication system after hand-off.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-252311